A cellulose acetate film is used in various photographic or optical elements because it has tough and enough flame retardant properties. The cellulose acetate film is used in a liquid crystal display (LCD). The cellulose acetate film has a high optical isotropy (a low birefringence) compared with other polymer films. Accordingly, the cellulose acetate film is usually used as an optical element requiring an optical isotropy, such as a protective film of a polarizing element or a color filter.
On the other hand, an optical compensatory sheet requires a high optical anisotropy (a high birefringence). Accordingly, a synthetic polymer film having a high birefringence, such as a polycarbonate film or a polysulfone film is usually used as the optical compensatory sheet.
Japanese Patent Provisional Publication Nos. 3(1991)-9325, 6(1994)-148429, 8(1996)-50206, 9(1997)-26572, U.S. Pat. Nos. 5,583,679 and 5,646,703 disclose another optical compensatory sheet, which comprises a transparent support and an optically anisotropic layer containing a discotic liquid crystal molecule. The high birefringence of the optical compensatory sheet is obtained by the optically anisotropic layer containing the discotic liquid crystal molecule. A cellulose acetate film is usually used as the transparent support, because the support preferably has a high optical isotropy (a low birefringence).
The above-mentioned optical compensatory sheet using the discotic liquid crystal molecule has usually been constructed to optically compensate a liquid crystal cell of a twisted nematic (TN) mode, which is used in a TFT liquid crystal display. If the optical compensatory sheet is used for a liquid crystal cell of another mode, such as a veritably aligned (VA) mode, an optically compensatory bend (OCB) mode or a hybrid aligned nematic (HAN) mode, the sheet cannot completely compensate the cell.
The optically anisotropic layer containing the discotic liquid crystal molecule can be used in combination with an optically anisotropic support. The optical anisotropy of the layer is combined with the optical anisotropy of the support to optically compensate the cell of the VA, OCB or HAN mode. A synthetic polymer film having a high birefringence, such as a polycarbonate film or a polysulfone film can be used as the optically anisotropic support. However, the synthetic polymer film is inferior as a support (physical properties and affinity with a coating layer). Therefore, the optically anisotropic support preferably is a lamination of the synthetic polymer film having a high birefringence and a cellulose acetate film, which is an excellent support, but has a low birefringence.
As is described above, a synthetic polymer film is used where the film should have an optical anisotropy (a high birefringence), while a cellulose acetate film is used where the film should have an optical isotropy (a low birefringence). This is a general principle in the technical field of an optical material such as an optical compensatory sheet.